Similarly, while AT&T's March quarter iPhone activations grew 69% year-over-year -- from 1.6 million in Q1 2009 to 2.7 million in Q1 2010 -- Apple's March quarter worldwide iPhone shipments grew 131% year-over-year. That represents worldwide growth that's more than twice as fast as U.S.-only growth.

To be sure, some of the lagging U.S. growth is probably seasonal -- we imagine that iPhone holiday gift-giving is more of a U.S. phenomenon than a rest-of-world phenomenon. And some of it could be a result of Apple adding more carrier partners in other countries. And some of it could just be that the iPhone business is maturing in the U.S.

But we think a lot of the lagging growth in the U.S. is a result of many peoples' reluctance to switch to AT&T, the only U.S. iPhone carrier, and stronger competition from other carriers' rival devices, including impressive Google Android-powered phones and cheap BlackBerry devices.

AT&T again said that more than one-third of iPhone activations were to customers who were new to AT&T. (And, again, that's a good number; likely more "switchers" than any other phone in the world is drawing.) But in context, more than two-thirds of U.S. wireless subscribers are still on carriers other than AT&T. And about half of those non-AT&T customers are Verizon subscribers.

So if Apple really wanted to maximize its U.S. iPhone sales -- which, given the platform-land grab-nature of today's smartphone market, it should want to do -- it needs to start adding more carrier partners as soon as its AT&T exclusive is up.

We believe it will do so, including Verizon, potentially as soon as this year.